Incandescent screen tube



Jan. '7, 1941- A. BouwERs INCANDESCENT SCREEN TUBE Filed 001:. 9, 1937 INVENTOR ALBE'RT BOUWER-S ATTORNEY Patented Jan. 7, 1941 UNITED STATES PATENT OFFICE INCAND'ESCENT SCREEN TUBE tion of Delaware Application October 9 1937, Serial No. 168,121

In Germany October 24, 1936 4 Claims.

Cathode ray tubes for the transmission of pictures are known wherein a thin metallic screen is brought to luminescence due toits being heated by electron bombardment. Such a screen will 5 hereinafter be referred to as an incandescent screen while a cathode ray tube comprising such a screen will be designated as an incandescent screen tube.

The operation of these tubes is for the rest similar to that of cathode ray tubes having a fluorescent screen. Like in the latter tubes a narrow modulated electron beam may be moved within the tube by suitable electrostatic or magnetic deflecting means so as to scan the whole of the area of the picture and to produce, in accordance with the modulation of the electron current, difierent degrees of brilliancy on the incandescent screen. With respect to cathode ray tubes having a fluorescent screen, incandescent screen tubes afiord the advantage that the picture on the incandescent screen may be of sufficient brightness to be projected enlarged on a projection screen by means of a lens system.

In the time available for a complete scanning of the incandescent screen each screen portion struck by cathode rays is heated during a very short period, the heating-up period; then, dur ing the remainder of the time, the cooling period, it cools again down to a determined temperature. If the temperature to which a portion after being repeatedly heated with the whole of the current intensity continuously returns lies far below the limit of visible incandescence, that portion of the energy supplied by the cathode ray beam by which in each scanning the material is firstly heated to said limit, is wasted for the picture production proper while the quality of the pictures is considerably reduced.

To obviate this drawback it has been proposed to raise the fundamental level of the temperature by an additional heating of the incandescent screen, for example, by means of an electric current or by irradiation. Such preliminary heating should never raise the temperature of the incandescent screen to a value higher than that which may have the dark portions of the picture. If the latter is; for example, 1300 Kelvin and the required temperature of incandescence is 2700 Kelvin the modulated cathode ray beam has still to bridge-over in the heating up period a temperature difference of 1400".

According to the invention, the temperature difference to be bridged-over in the heating up period is reduced to a much higher extentthan is possible with the known preliminary heating.

For this purpose the preliminary heating of the incandescent screen is not simultaneously efiected for all the portions of the screen but is brought about by a, second cathode-ray beam which precedes the picture producing beam and which, in scanning the incandescent screen, keeps step with the latter beam.

This auxiliary beam raises the screen material to a temperature such that without further heating the screen still just appears dark to the eye of the observer. It should preferably have a larger cross sectional area than the picture producing beam in order to ensure that the latter less easily leaves the trace of the auxiliary beam.

An incandescent screen tube according to the invention comprises an electrode system which produces two movable cathode ray beams of which one may be modulated without its modula ing acting upon the nature of the other. One of the cathode ray beams brings about, for example, a jump in the temperature of the parts of the incandescent screen which are struck by it, from 1300 K. to 1900 K. while with the highest intensity the modulated beam further raises the temperature, for example, to 2700 K. and consequently has to bridge-over only a temperature difference of 800.

When the modulated beam is switched-off, the remaining brilliancy of the screen which is perceptible to the eye does not correspond to the temperature of 1900 K. but is much less because the beam of rays touches the parts only during a very short time and subsequent to the load cooling down sets in again at once. With this wavefront like heating of the incandescent screen, it is therefore possible to preheat the parts to be struck by the picture producing beam to a higher temperature than when all parts are simultaneously heated.

In the incandescent screen tube according to the invention, the picture producing beam of rays'may consequently have not only less energy than in the case where preliminary heating does not take place at all, but also less energy than would be required in the case of continuous heating.

Furthermore, it has been found that the material can better withstand such heating in stages than a direct and more rapid heating with an intenser load by the modulated beam alone so that owing to the application of the invention the life of the tube is increased.

The invention will be explained more fully with reference to the accompanying drawing which represents, by way of example, one embodiment thereof.

Figure 1 represents diagrammatically a sectional view of an incandescent screen tube according to the invention.

Figure 2 represents part of the tube shown in Fig. 1.

The tube comprises a glass bulb l which has the shape of a cylinder with a funnel-like widened portion as is usual in Braun tubes. The cylindrical bulb portion contains the electrodes for the production of two narrow directed electron beams. The one beam, which has to produce the picture, is emitted by an incandescent cathode 2. The modulation of this beam is brought about by an electrode 3 while an electrostatic lens system consisting of a perforated disc 4 and of two cylinders 5 and 6 serve the purpose of focussing and of electron acceleration.

The second beam is emitted by an incandescent cathode 22. As it need not be modulated, the electrode 23 is connected to the cathode 22. For the purpose or focussing is likewise provided an electrostatic lens system consisting of a disc 24 and of two cylinders 25 and 26.

For either of the two cathode ray beams the tube consequently contains a source of electrons and a collecting and accelerating device, the cathode ray beams being thus prevented as far as possible from mutually influencing one another. Above all it is excluded that the variations in the current intensity of the modulated beam become perceptible in the unmodulated beam.

That tube portion which is located in the drawing to the right of the line AB has to be imagined to be turned by 90 with respect to the axis of the tube so that the axes of the cylinders 5 and 25 are not located in the plane of the drawing but in a plane at right angles thereto.

By a magnetic system, not shown in the drawing, the two cathode ray beams may be deflected in a direction perpendicular to the plane of the drawing. Deflecting plates 1 and 8 serve for the deflection in a direction parallel to the plane of the drawing. The wall of the tube is provided with the usual silver mirror 9 which is connected at ID to a current supply conductor.

In the funnel-shaped tube portion is arranged a metallic frame H to which an incandescent screen I2 is secured at its edges. A current supply conductor I3 is led to the exterior of the glass bulb through an extension M. The current supply conductors of the cathodes 2 and 22 and of the electrodes 3, 4, 23 and 24 are hermetically sealed into a pinch IS. The current supply wires of the electrodes 5, 6, 25 and 26 and of the deflecting plates 1 and 8 are laterally passed out of the tube through the wall.

In a manner known in itself the narrow cathode ray beams produced and focused by the electrode system and which strike the incandescent screen l2 at two adjacent points, may be moved within the tube in such manner that the points of impact of the beams move on the screen in parallel lines, scanning lines, pass over the whole of the screen in a lapse of time, for example, of 0.04. and then start anew on their way over the screen so that the latter is scanned 25 times in one second. The beam emitted by the cathode 2 lays behind the beam emitted by the cathode 22 at the slightest possible distance. When moving back, the current is, as is cus- 'tomary interrupted by a negative potential applied, for example, to the cylinders 5 and 25.

The modulated beam has at the screen a diameter, for example, of 0.3 mm. whereas the unmodulated beam may have a larger diameter, for example, twice as large.

The unmodulated beam raises each incandescent screen portion on which it impinges, to incandescence, but not to a degree such that this incandescence is perceived by the eye as a disturbing lighting up of the screen. Accordingly,

I as the current intensity of the modulated beam is higher or lower, each screen portion which is preheated already by the auxiliary beam is raised by the picture beam to a higher temperature of incandescence with the highest current intensity that may occur and with an incandescent screen of tungsten, for example, to a temperature of 2700 Kelvin. Owing to a. suitable modulation of the picture producing beam by means of the control electrode 3 the brilliancy varies in such manner that on the screen a luminous picture is produced. By means of an optical system this picture may be projected from the incandescent screen in a direction turned away from the cathodes onto a projection screen. The incandescent screen may be constituted by a closed tungsten foil having a thickness, for example, of 1 micron or by a perforated foil of tungsten or other highly refractory metal hav ing a greater thickness up to 6 microns as has been indicated in a patent application of the same applicant filed May 17, 1936, Serial No. 81,991. The invention is, however, not restricted toa determined shape or nature of the incandescent screen.

The auxiliary beam must be so directed that its point of impact with the screen lies close before the point of impact of the lagging beam. Correcting means are preferably provided to be enabled to alter the situation of one of the points of impact. In the example of construction, the cylinder 26 is divided to that end, as may be seen from Fig. 2, into quadrants which are separated by small insulators l6, l1, l8 and I9. Each quadrant has its own current supply conductor so that small potential differences may be applied which allow of giving the cathode ray beam the desired direction.

What I claim is:

1. In a television receiving apparatus, means for receiving signals representative of the optical image to be reproduced, a cathode ray tube including an envelope containing an incandescing screen, means for scanning said screen with a cathode ray having substantially constant value for heating elemental sections of said screen to a predeterminable value sequentially, and cathode ray means for scanning said incandescent screen in substantially timed relationship to said heating subsequently to the sequential heating of said elemental sections.

2. In a television receiving apparatus, means for receiving signals representative of the optical image to be reproduced, a cathode ray tube including an envelope containing an incandescing screen, means for scanning said screen with a cathode ray having substantially constant value for heating elemental sections of said screen to a predeterminable value sequentially, and means for reproducing the optical values of said optical image, said latter means comprising a cathode ray which scans the incandescing screen simultaneously with the cathode ray of substantially constant value, said scanning rays being spaced in point of impact by a predeterminable distance.

3. In a television receiving apparatus, means for receiving signals representative of the optical image to be reproduced, a cathode ray tube including an envelope, incandescing means responsive to electron bombardment positioned within said envelope, means for developing, accelerating,

deflecting, and modulating a cathode ray beam, the modulations of said beam being in accordance with the received signals, means for developing an auxiliary electron beam, means for deflecting said auxiliary beam whereby said auxiliary beam sequentially traverses the same elemental areas as the modulated beam, and means for focussing impingement with the screen than the beam 10 which reconstructs the optical image.

ALBERT BOUWERS. 

